1. Field of the Invention
The present invention relates to bipolar transistors. In particular, the present invention is an improved bipolar transistor which includes a thin barrier layer of a wider band gap semiconductor material between the base and emitter.
2. Description of the Prior Art
The desire for higher speed bipolar transistors has led to the investigation of various transistor structures, and to the development of bipolar transistors using semiconductor materials other than silicon. Particular emphasis has been placed on devices using gallium arsenide (GaAs). Among the advantages offered by gallium arsenide over silicon transistors are the higher electron mobility of gallium arsenide, the availability of semi-insulating substrates, and expected superior radiation hardness and high temperature performance.
Heterojunction bipolar transistors (HBT), which typically use a wide band gap aluminum gallium arsenide (AlGaAs or Al.sub.x Ga.sub.1-x As) emitter, offer several potential advantages over homojunction GaAs devices for high speed applications. In a AlGaAs/GaAs heterojunction bipolar transistor, the wide band gap AlGaAs emitter introduces an extra barrier for minority carrier injection from base to emitter. As a result, the emitter efficiency can be very high and nearly independent of the doping density of the base. As a result, the base can be doped heavily to reduce base resistance, without sacrificing emitter injection efficiency. Description of heterojunction bipolar transistors can be found, for example, in the following papers: H. Kroemer, "Heterostructure Bipolar Transistors: What Should We Build?", J. Vac Sci. Technol., Bl(2), pp. 126-130, April-June 1983; N. Chand and H. Morkoc, "Doping Effects and Compositional Grading in Al.sub.x Ga.sub.1-x As/GaAs Heterojunction Bipolar Transistors", IEEE Transactions on Electron Devices, Vol. Ed-32, No. 6, pp 1064-1068, June 1985; A. Grindberg, M. Shur, R. Fischer and H. Morkoc, "An Investigation of the Effect of Graded Layers and Tunneling on the Performance of AlGaAs/GaAs Heterojunction Bipolar Transistors", IEEE Transactions on Electron Devices, Vol. Ed-31, No. 12, pp 1758-1764, December 1984; P. Asbeck, D. Miller, R. Milano, J. Harris, Jr., G. Kaelin and R. Zucca, "(Ga,Al)As/GaAs Bipolar Transistors For Digital Integrated Circuits", IEDM 81, pp 629-632, 1981.
Despite the advantages, AlGaAs/GaAs heterojunction bipolar transistors also have significant shortcomings. In particular, the AlGaAs emitter has several disadvantages related to the traps associated with the dopants, high contact resistance (typically much larger than for comparably doped GaAs), and higher series resistance because of the low mobility and electron velocity in AlGaAs.